The performance of an electrochemical cell, such as a proton exchange membrane (PEM) fuel cell and a direct methanol fuel cell (DMFC), is largely determined by the membrane-electrode assembly (MEA). The MEA has an anode for fuel oxidation, a cathode for oxygen reduction, and an ion-conducting membrane for proton conductance.
The fuel oxidation reaction and the oxygen reduction reaction of these cells have typically slow kinetics. Therefore, catalysts such as platinum and its alloys are often used to catalyze these reactions.
Catalysts are generally made into porous layers in order to increase the surface contact area between the reactants and the catalyst particles. The layers can be applied either to the membrane or to a gas diffusion medium. A catalyst electrode (hereinafter “electrode”) is fabricated by applying the layer to the gas diffusion medium. A catalyst layer that is applied to a membrane produces a catalyst-coated membrane (CCM). Normally, both sides of the membrane are coated.
In order to reduce the cost of a fuel cell, it is desirable to reduce the amount of noble metal catalysts in the catalyst layers. Initially, the minimum catalyst loading in an electrode able to provide good performance was found to be over 4.0 mg/cm2. Subsequently, metal nano-particles having a higher surface area were prepared upon a carbon black support layer, as illustrated in U.S. Pat. No. 4,166,143, granted to Petrow et al on Aug. 28, 1979; 4,876,115, granted to Raistrick on Oct. 24, 1989; and Re. 33,149 granted to Petrow et al on Jan. 16, 1990.
Both electrons and protons are involved in fuel cell reactions, thus requiring both electrical, as well as proton conductivity. In a traditional MEA this will limit the reaction zone within the interface, between the catalyst layer, and the ion-conducting membrane. This interfacial region is extremely thin and the total surface area of the catalyst particles in this region is low. Therefore, the catalyst layer is unable to provide a high current density. The catalyst that is not in contact with the membrane is simply wasted.
Nafion, an ionic-conducting, perfluorinated ionomer can be incorporated into the catalyst layer to improve the current density. Incorporating Nafion into the entire catalyst layer will provide improved conduction for protons.
Nafion® can be impregnated into a catalyst layer by brushing and spraying, or by respectively floating, or dipping the electrode into a Nafion solution. Ticianelli et al, Methods to advance technology of proton exchange membrane fuel cells, J. Electrochem. Soc. pp. 2209-2214 (1988), September. Poltarzewski et al, Nafion distribution in gas diffusion electrodes for solid polymer-electrolyte-fuel-cell applications, J. Electrochem. Soc. pp 761-765 (1992), March. Applying Nafion in this way provides an opportunity to incorporate a water-repelling agent such as polytetrafluoroethylene (PTFE) into the catalyst layer. The final catalyst layer will be controllably hydrophobic and be able to reduce the likelihood of flooding. The disadvantage of this method is that it is very difficult to control the amount of Nafion applied. Therefore, it is impossible to have a homogeneous distribution of Nafion over the entire catalyst layer.
Another method of incorporating Nafion into a catalyst layer is to mix catalysts, especially supported catalysts, directly with Nafion and then use the resulting mixture to fabricate the catalyst layer, as illustrated in U.S. Pat. No. 5,211,984, granted to Wilson on May 18, 1993; 5,723,173, granted to Fukuoka et al on Mar. 3, 1998; 5,728,485, granted to Watanabe et al on Mar. 17, 1998; and 6,309,772, granted to Zuber et al on Oct. 30, 2001. Mixing the catalysts and Nafion forms a mixture providing an even distribution through the entire catalyst layer. Solvents such as glycerol may be used during the mixing in order to achieve a preferential viscosity and to hold the catalyst particles in suspension in order to minimize their agglomeration, as shown in U.S. Pat. No. 5,211,984, granted to Wilson on May 18, 1993.
Sometimes the Nafion solution is converted into a colloid suspension by adding a proper organic solvent before mixing it with the catalyst, as illustrated in U.S. Pat. No. 5,723,173, granted to Fukuoka et al on Mar. 3, 1998. It is described in this patent that colloidal Nafion forms a good network that achieves a uniform distribution with the catalyst particles.
Directly mixing the Nafion solution, or colloid suspension with the catalyst, however, makes it difficult to incorporate PTFE into the catalyst layer. This is because PTFE normally needs to be sintered at a temperature higher than 330° C. However, such a temperature will destroy Nafion. Unfortunately the catalyst layer is more likely to be flooded without the PTFE.
All these recent developments have helped to decrease the catalyst loading from 4.0 mg/cm2 or more, to 0.5 mg/cm2 or less. However, a fuel cell that has slightly loaded catalyst electrodes has much lower performance than a fuel cell using highly loaded catalyst electrodes. This lowered performance results from there being many catalyst sites that will not become active and are figuratively “dead.” The reasons for this could be: the reactant cannot reach the catalyst sites because they are blocked; the Nafion near these catalyst sites cannot be easily hydrated; or the ionic or electronic continuity is not established at these sites. Catalyst sites that cannot participate in the electrochemical reactions are “dead”.
The present invention provides a procedure and an article made by the procedure whereby catalyst utilization is enhanced significantly.
The present invention has discovered that “dead” catalyst sites can be activated by treating the catalyst layer with steam or a high temperature solution.